Bimodal star-block copolymers are well known in the field of anionic polymerization. These copolymers are generally prepared forming first a linear block-copolymer bearing active lithium atoms on one end of the chain; these linear (active) polymeric chains are then coupled, by addition of a polyfunctional compound having at least three reactive sites able to react with the C--Li bond of each polymeric chain, by coupling the polymeric chains to the functional groups of said compound.
U.S. Pat. No. 3,280,084 discloses the polymerization of butadiene with buthyl-lithium as initiator to form blocks of polybutadiene ending with lithium, which (after having been coupled with divinyl-benzene) gives rise to star copolymers having a poly-divinyl-benzene nucleus and several identical polybutadiene arms departing therefrom; these arms can be styrene-butadiene copolymers of the "random" or "block" type.
U.S. Pat. No. 3,639,517 discloses star-block copolymers where the side arms attached to the same nucleus have a different molecular weight.
These arms are formed by using differential additions of styrene monomer and of initiator to give S--Li, S.sub.1 --Li and S.sub.2 --Li, where S, S.sub.1 and S.sub.2 are polystyrene blocks having a different molecular weight, then adding butadiene and finally coupling these arms to give star-block copolymers having a bi-, tri and poly-modal distribution of the molecular weight.
U.S. Pat. No. 4,086,298 discloses star-block copolymers having a mixture of arms, some of which are formed by polymerizing styrene with alkyl-lithium to form polymeric S--Li blocks and then adding a styrene-butadiene mixture to form a "tapered" polymer represented by S--B.fwdarw.S.sub.1, wherein the arrow represents a "tapered" segment. Other arms consist of one single butadiene-styrene tapered copolymer segment (B--S.sub.1).
U.S. Pat. No. 4,221,884 describes star and bimodal block copolymers, that are transparent and impact resistant, expecially at the low temperature, such as between -20.degree. and -40.degree. C., having general formula: EQU (S--S'/B--B').sub.m --X--(B'--B/S').sub.n (I)
where S is a non elastomeric block of 80-95% by weight of the total mono-vinyl aromatic compound, S'/B or B/S' is a block copolymer of "random" and/or "tapered" type of the mono-vinyl aromatic compound and of the conjugated diene, wherein the S'/B weight ratio is lower than 1, B' is an elastomeric segment of conjugated polydiene, containing 20-40% by weight of the total conjugated diene, m and n are integers comprised between 3 and 20 and X is the radical of a polyfunctional coupling agent forming the nucleus of the star-block copolymer. These star-block copolymers, although endowed with very good high impact features, are not wholly satisfactory as to their optical properties.
The Applicant has now obtained bimodal star-block copolymers having excellent optical features, combined with satisfactory high impact features, by introducing in the copolymer a structural modification and reducing the amount of conjugated diene in the B' segment of the general formula (I).